1. Field of the Invention
This invention relates to a permanent magnet-type lifting device and more particularly to a permanent magnet type lifting device adapted to lift steel articles or steel materials.
2. Description of the Prior Art
A conventional permanent magnet-type lifting device of this general type will be described with reference to FIGS. 1 through 3.
FIG. 1 is a front view of such a lifing device while FIGS. 2 and 3 are sectional views of the lifting device showing different positions of the permanent magnet.
The conventional permanent magnet-type lifting device as shown in FIGS. 1 through 3 includes yokes 1A and 1B made of a magnetically permeable material and, non-magnetic members 2A and 2B which form a single unit with the yokes 1A and 1B, the signle unit having a circular hole 3 in which a permanent magnet 4 is rotatably mounted. The permanent magnet 4 may be turned by a handle 5. The yokes 1A and 1B have ends 6A and 6B, respectively.
When the permanent magnet 4 is turned until its N and S poles are positioned as shown in FIG. 2, then the yokes 1A and 1B are magnetized so that magnetic poles N and S are formed at the yoke ends 6A and 6B, respectively. Thus, an object such as a steel article or a steel material can be attracted by the lifting device. When the permanent magnet is further turned until its N and S poles are positioned as shown in FIG. 3, the attracting force is eliminated.
Such a conventional permanent magnet type lifting device is disadvantageous in the following points. As the permanent magnet 4 is an integral cost magnet, its manufacturing cost is relatively high. Furthermore, it is necessary to machine the two opposite end faces of the permanent magnet with high accuracy in compliance with the sliding surface of the circular hole 3 which is formed by the yokes 1A and 1B. Accordingly, the machining cost is again relatively high while the productivity of the lifting device is low.
Also, known in the art is a permanent magnet-type lifting device in which plurality of permanent magnets are provided between magnetic members in such a manner that they are simultaneously turned by an associated means. Such a device is described in U.S. Pat. No. 3,452,310 issued June 24, 1969 to Israelson.
To release an object which has been lifted by a conventional lifting device, if the magnetic forces of the permanent magnets arranged in two lines are not equal to each other, then the resulting effective magnetic field in the release position remains non-zero at the attracting surface even through the direction of magnetic force of the upper permanent magnet is made opposite to that of the lower permanent magnet so that the magnetic forces cannot be entirely cancelled by each other. Thus, with the conventional lifting device, it is difficult to positively release the object lifted thereby.
A permanent magnet-type lifting device which has eliminated such difficulty has ben proposed in the art. However, there still remains a problem to be solved. That is, its attracting surface is flat and hence its attaching force is insufficient.
Still further, a permanent magnet-type lifting device with an improved attracting force is also known in the art. In this lifting device, the permanent magnet itself must be subjected to spherical machining which is quite difficult to do.